It has been proposed to feed liquid to an applicator with an apparatus consisting of an inner liquid container in which the liquid is supplied to the user and an outer pressure vessel. Although the provision of an inner and outer container may at first sight seem unnecessary such a two container system has been found to possess various advantages: for example, it makes the system cleaner to use and it makes cleaning of the system after use easier.
In our International patent application, publication number WO 80/00315 published Mar. 6, 1980, and corresponding to our later published U.S. Pat. No. 4,422,788, an apparatus for applying liquid to a surface is described. In one of the embodiments described the apparatus has an inner paint container and an outer pressure vessel and in use pressurized gas is injected into the interior of the outer vessel, which is sealed, and passed into the inner paint container and expels paint from the inner container through a dip tube to an applicator connected to the apparatus by a flexible tube. It is proposed that paint be supplied to the user in the inner paint container which the user inserts into the outer vessel before use. Since it is proposed that the paint be supplied to the user in the inner paint container, it is important that this container be of relatively simple and cheap construction.
In U.S. Pat. No. 3,776,645 an apparatus is described in which an outer container is provided into which an open liquid container may be inserted. Liquid in the open topped container is expelled through a tube to an applicator by pressure generated by a squeeze bulb.
In U.S. Pat. No. 3,640,630 an apparatus is described including an outer vessel housing an open topped container in which a flexible plastics bag containing paint may be inserted. In order to regulate the pressure in the inner container, the container forms a seal with a wall of an outer vessel and when the pressure exceeds a threshold value, the seal is broken until the pressure returns to below the threshold value.
We have found that in order to ensure satisfactory feeding of the paint to the applicator and satisfactory application by the applicator the paint should possess special physical properties. It is therefore desirable that the apparatus be able to be used only with paint which has been designed specifically for use with the apparatus. The use of an unsuitable liquid may also damage the apparatus.
With the apparatus of U.S. Pat. Nos. 3,776,645 or 3,640,630, it would be possible for a user to take a conventional can of paint, remove the lid and place the can in the apparatus, or alternatively pour paint into the outer container. The apparatus could then be operated in the usual manner using the conventional paint.
Similarly, in the embodiment of our International patent application described above, it would be possible for a user to place a conventional paint can, instead of the inner paint container containing special paint, inside the outer vessel, or alternatively pour paint into the outer vessel. The apparatus therefore also has the disadvantage that it can be used with unsuitable paint. Although there is a reference in International patent application No. WO 80/00315 to providing means for preventing the insertion of an unsuitable container this would not prevent paint being poured directly into the outer vessel.
If any of the apparatus described above is used in these ways, then the apparatus could become defective or even dangerous as a result of paint penetrating to parts of the apparatus that it is not intended to reach. At the very least the apparatus would require extensive cleaning after such use.
Problems of safety can to some extent be overcome by introducing safety valves and similar features but there always remains the possibility of failure of these various safety mechanisms. It is therefore desirable that, even in the event of such safety mechanisms not taking effect, and the pressurized vessel bursting, the user is not seriously affected.